Excitonic order at strong coupling: Pseudospin, doping, and ferromagnetism

A tight-binding model is introduced to describe the strong interaction limi t of excitonic ordering. At stoichiometry, the model reduces in the strong- coupling limit to a pseudospin model with approximate U(4) symmetry. Excito nic order appears in the pseudospin model as in-plane pseudomagnetism. The U(4) symmetry unifies all possible singlet and tripler order parameters des cribing such states. Superexchange, Hunds-rule coupling, and other perturba tions act as anisotropies splitting the U(4) manifold, ultimately stabilizi ng a paramagnetic tripler state. The tendency to ferromagnetism with doping (observed experimentally in the hexaborides is explained as a spin-flip tr ansition to a different orientation of the U(4) order parameter. The physic al mechanism favoring such a reorientation is the enhanced coherence land h ence lower kinetic energy) of the doped electrons in a ferromagnetic backgr ound relative to the paramagnet. A discussion of the physical meaning of va rious excitonic states and their experimental consequences is also provided .